Electrophotographic printing is well known and has been widely refined. For example, today, almost every office and indeed some homes have electrophotographic copiers. The industry has grown to the point where it is now a highly competitive multi-billion dollar industry. In most instances, these home and office copiers are capable of providing only about a few copies per minute.
In electrophotography, images are photoelectrically formed on a photoconductive layer mounted on a conductive base. Liquid or dry developer or toner mixtures may be used to develop the requisite image.
Liquid toner dispersions for use in the process are formed by dispersing dyes or pigments and natural or synthetic resin materials in a highly insulating, low dielectric constant carrier liquid. Charge control agents are added to the liquid toner dispersions to aid in charging the pigment and dye particles to the requisite polarity for proper image formation on the desired substrate.
The photoconductive layer is sensitized by electrical charging whereby electrical charges are uniformly distributed over the surface. The photoconductive layer is then exposed by projecting or alternatively by writing an image over the surface with a laser, L.E.D., or the like. The electrical charges on the photoconductive layer are conducted away from the areas exposed to light with an electrostatic charge remaining in the image area. The charged pigment and/or dye particles from the liquid toner dispersion contact and adhere to the image areas of the photoconductive layer. The image is then transferred to the desired substrate, such as a travelling web of paper or the like.
In contrast to office and home copiers, high speed electrophotographic printing presses are being developed wherein successive images are rapidly formed on the photoconductive medium for rapid transfer to carrier sheets or the like travelling at speeds of greater than 100 ft./min. and even at speeds of from 300-500 ft./min. As can be readily understood, in such high speed methods and devices, to provide a commercially viable product, it is desirable to accurately charge the photoconductive surface adequately so that image formation will be of a high quality, uniform nature. As such, it is desirable to provide a control system to ensure that the desired, predetermined charge is imparted to the rapidly rotating photoconductor despite irregularities that may occur, for instance, due to irregular "out of round" cylinders, photoconductor deterioration, or other causes.
Such control systems are not, per se, new. However, the known prior art systems were used in conjunction with office copiers that could not meet the high speed requirements of the high speed electrophotographic printing press herein contemplated and they did not provide feedback control to a plurality of charge coronas.
It is also desirable to provide a support mechanism for the electrical charging means that ensures proper spacing of the corona discharge wires above the photoconductive cylinder and is easily detached from the printing press for repair and maintenance.